1. Field of the Invention
This invention concerns an optical scanning apparatus which converts optical information obtained by receiving reflected light after illuminating an object carrying optical information to a corresponding electrical signal.
2. Description of the Related Art
Up to now, an optical scanning apparatus which illuminates an object to be read with laser light or the like and converts the optical information to a corresponding electric signal by receiving the reflected light therefrom has been used in optical information readers which are used for obtaining optical information from objects that carry optical information such as bar codes.
An optical scanning apparatus of the same type disclosed in Japanese Patent Laid-Open Publication No. Hei. 3-179580 which integrates an optical scanning mirror for illumination scanning and a light receiving mirror for receiving the reflected light has been designed to miniaturize the optical scanning apparatus.
In other words, as shown in FIG. 11, a conventional optical scanning apparatus used in a bar code reader 82 to read the bar code 80a from a bar code label 80 includes a light source 84 which generates light to illuminate the bar code 80 as a parallel beam through a collimating lens 84a, a flat optical scanning mirror 86 for optically scanning the bar code 80 by reflecting light from the light source 84, a light receiving element 88 which receives light reflected from the bar code label 80 when the bar code is optically scanned and then converts it to an electrical signal, a light receiving convex mirror 90 for directing the reflected light from the bar code label 80 to the light receiving element 88, and a light receiving lens 92 for focusing the light that was directed to the light receiving element 88 by the convex plane mirror 90 on the light receiving element 88. By integrating the flat mirror 86 and the convex plane mirror 90 and fixing them on the shaft of a motor 94, one known optical scanning apparatus which can simultaneously control optical scanning and light receiving through the rotation of the motor 94 is realized.
In FIG. 11, numeral 96 refers to a window formed in the bar code reader 82 through which the light for illuminating the label 80 and the light reflected from the bar code label 80 passes. Numeral 98 refers to a switch which activates the optical scanning apparatus to read the bar code 80a.
For the conventional optical scanning apparatus described above, light generated by the light source 84 and passing through the collimating lens 84a is reflected by the plane mirror 86 and radiated outside after passing through the window 96. Since the flat mirror 86 oscillates sideways in accordance with the rotation of the motor 94, a scanning line is formed as a result of this oscillation. Therefore, if objects to be read such as the bar code 80 and the like are placed along this scanning line, the light for illumination would be reflected irregularly from the objects to be read. Then, the irregularly reflected light reaches the convex mirror 90 through the window 96 and is directed by the convex mirror 90 to the light receiving element 88 and also is directed to the light receiving point of a light receiving element 88 through the light receiving lens 92. During this time, since the convex mirror 90 oscillates together with the plane mirror 86, the reflected light is always directed in the direction of the light receiving element 88 and thus, light is received efficiently by the light receiving element 88.
For the conventional optical scanning apparatus described above, because the plane mirror 86 for optical scanning and the convex mirror 90 for receiving light are oscillate together using only one motor 94, optical scanning of the object to be read and the receiving of the reflected light can be done performed efficiently and in addition, simplification of the structure of the apparatus is possible. However, by convention, since the convex mirror 90 with the plane mirror 86 mounted thereon oscillates responsive to the motor 94, there is a significant load applied to the rotational axis of the motor 94 and thus, one problem is that the plane mirror 86 and the convex mirror 90 cannot oscillate at high speeds using the motor 94.
Moreover, even if substrates formed beforehand to correspond to the plane mirror 86 and the convex mirror 90 which have surfaces plated with aluminum, gold or the like are used to reduce the load applied to the motor 94, the reflecting mirror member is large in size and thus, a relatively large motor 94 must be used and simply reducing the weight of the reflecting mirror member would not make it oscillate faster.
In other words, with conventional optical scanning apparatus, because of limits due to the characteristics of the motor, the weight of the mirror and the like, optical scanning, while possible at rates of tens of Hz, is impossible at high speeds such as tens of kHz.
In addition, because there is a need to install a large reflecting mirror on the rotation axis of the motor 94, a heavy load is applied to the rotation axis when it receives shocks from the outside and thus, there are problems with its shock and vibration resistance.
Furthermore, in the prior art, since the reflecting mirror, which is used for optical scanning and light receiving, oscillates due to the rotation of the motor, the scanning direction is limited to the one-dimensional direction that corresponds to the direction of the rotation of the motor and thus, for example, it was not possible to change the illumination point in the one-way optical scanning for reading each label of a multiple-row bar code label. In other words, while the conventional optical scanning apparatus can displace the reflective mirror for optical scanning and light receiving in a one-dimensional direction, it cannot perform displacements in a two-dimensional direction. As used herein, "one dimensional direction" means mirror displacement along one axis thereof, while "two dimensional direction" means mirror displacement along two mirror axes, e.g., x and y axes.